The Ia alloantigens are molecules determined by the I (immune response) region of the major histocompatibility complex. We are studying the chemical and serological properties of immune Ia molecules which most probably have an active role in the genetic control of the immune response. Detailed structural studies of molecules determined by different I region genes, and by different alleles of the same I region gene will be carried out using isoelectric focussing and tryptic peptide analysis. These techniques will be applied to radiolabeled Ia molecules isolated by immune precipitation and separated into their component glycopolypeptide chains by chromatography or electrophoresis. Serological analysis will be directed toward definition of the number of Ia molecules and the alloantigenic complexity of individual Ia molecules. These data will help determine whether the diverse traits mapping in the I region are actually distinct or have common mechanisms mediated by the same molecule.